But as we saw, he HAD no former personality to revert to.
Or then he had a very bland one.
No, seriously. We have been told many times that the Borg don't procreate, not the usual way - and apparently not in any unusual way, either, as there has never been actual evidence of the Borg procreating. Granted, the Borg (or Q, or Guinan) may lie about such things, but it's just as easy to assume that they speak the truth, and the only way to become Borg is to get assimilated.
It's just that the Borg also assimilate children, including infants. If Hugh were assimilated at the age of two months, he'd hardly have a "personality"...
Remember why the Borg assimilate. They think they are doing good to the assimilees. If they cloned Drones all by themselves, that would mean less people assimilated and thus less good done. Plus, we don't know that they would actually need the Drones for any purpose other than enjoying being a Drone.
Timo Saloniemi